1. Field of the Invention
The present invention relates to the field of data collection devices. In particular, the present invention relates to a method and apparatus for measuring electrical characteristics (e.g., current, voltage, phase, etc.) between a radio frequency (RF) power source and a load at a set of harmonic frequencies to determine information about the load (e.g., load impedance, power dissipation, etc).
2. Background Information
In many applications in which a load is powered by an RF signal provided by a source, it is useful, and sometimes necessary, to determine certain load parameters (e.g., load impedance, power dissipation, etc). For example, devices such as impedance matching networks in RF powered systems generally attempt to accurately detect voltage, current, power, and/or phase in the RF powered system to match impedance between an RF power source and a load, and thereby provide maximum power transfer to the load. An impedance matching network is often necessary, for example, in an RF powered plasma processing system utilizing an RF power generator (source) and a plasma chamber (load) to ensure that a desired power delivery level is matched and maintained for the plasma chamber.
Unfortunately, in some applications, monitoring load parameters with desired accuracy may be relatively difficult. Nonlinear voltage-current relationships of some types of loads and variable frequency of a power signal provided by a power source can make determining load parameters relatively difficult. In the case of RF powered plasma processing systems, for example, the dynamic impedance of the plasma chamber may produce harmonics, which, in turn, can return from an input of the plasma chamber to produce relatively substantial errors in diagnostics/measuring devices and/or impedance matching networks. In particular, the plasma chamber may produce harmonics as its impedance changes and stray current is discharged due to such factors as plasma chamber pressure, temperature, chemical composition, plasma ignition, chamber dimension, etc. Without taking harmonics into consideration, information about a load (e.g., a plasma chamber) may be incomplete and/or inaccurate.
In some applications, relatively complete and accurate information regarding an RF powered load is desired. For example, in the case of RF powered plasma processing systems, information about the plasma chamber, such as impedance and/or power transfer at fundamental and harmonic frequencies, enables impedance matching networks to operate optimally. Furthermore, characteristics of the plasma chamber at various harmonics could be used as "fingerprints" to allow identification/repetition of particular plasma processes.
In the past, data collection devices for applications such as RF powered plasma processing systems have not been able to provide desired accuracy in the presence of harmonics or have been relatively expensive and difficult to implement. For example, some prior art data collection devices collect data directly from a plasma chamber by using a measuring probe that is inserted into the plasma chamber or is part of a wall of the plasma chamber. However, such prior art data collection devices are intrusive (i.e., they may affect plasma chamber conditions), and thus, relatively difficult to implement in some instances since the probe must be directly inserted into the plasma chamber. Furthermore, such prior art data collection devices are relatively expensive since they generally must utilize relatively complex measuring probes inside the plasma chamber to detect chemical properties and perform relatively complex calculations to determine information about the plasma chamber. Other prior art data collection devices are not capable of measuring impedance/power dissipation of a load at harmonic frequencies.
Therefore, what is desired is a relatively inexpensive, transparent (in situ) system for interfacing with any number of RF powered systems for determining information about an RF powered load (e.g., load impedance, power dissipation, etc.) even in the presence of harmonics.